1. Field of the Invention
The present invention relates to a punch press drive device that drives a punch with a servomotor as a drive source.
2. Description of the Related Art
In order to decrease the noise and increase the processing quality on a punch press, the changing of the punch speed mid-stroke has been requested. A device where the punch speed can be changed has been realized on each type of hydraulic press but the equipment necessary for hydraulic systems are complicated and expensive. Due to this, instead of a hydraulic press type system, a system using a servomotor has been developed as a punch press drive device having a simple arrangement which is capable of changing the punch speed mid-stroke.
A three phase type induction motor as shown as a primary winding in FIG. 4A is used as this servomotor. This type of servomotor has output properties as shown by the curve p in FIG. 4B where the output (watts) is proportional to the speed (rotation count). When this is used for driving a punch press, it is used in a speed range from the motor speed N.sub.L ' where the minimum output W.sub.L ' necessary for accurate punch processing and is possible and is obtained to the maximum motor speed Nb' where the linearity of the output curve p is maintained.
However, in punch processing, high speed punch actions are required for a nibbling process, or the like where shearing of the work is performed by continuous punching and an increase in speed of the punch processing is difficult in the aforementioned speed range (N.sub.L '-Nb'). If the servomotor is such that the proportion of output change to the speed change is small (the slope of the curve p of FIG. 4B is less steep), high speed processing becomes possible but the output at low speed is insufficient.